The present disclosure relates to a storage element which has a plurality of magnetic layers and which performs recording using spin torque magnetization reversal and to a storage device.
Concomitant with significant development of various types of information apparatuses from mobile terminals to large scale servers, in elements, such as memory and logic elements, forming the above apparatuses, further improvements in performance, such as increase in degree of integration, increase in operation speed, and reduction in power consumption, have been pursued. In particular, the progress of semiconductor nonvolatile memories is remarkable, and flash memories each functioning as a large scale file memory have increasingly spread so as to push out hard disk drives from the market.
In addition, in order to replace NOR flash memories, DRAMs, and the like, which have been commonly used, development of a ferroelectric random access memory (FeRAM), a magnetic random access memory (MRAM), a phase-change random access memory (PCRAM), and the like has been carried out for code storage application and for further application to working memories. Some of those memories mentioned above have already started to be used in practice.
In particular, since the MRAM performs data storage using a magnetization direction of a magnetic substance, rewriting can be practically performed an approximately infinite number of times (1015 times or more) at a high speed, and the MRAM has already been used, for example, in the fields of industrial automations and airplanes.
Because of the high-speed operation and reliability, the MRAM has been expected to be further used for code storage and working memory applications; however, in fact, reduction in power consumption and increase in capacity are subjects to be overcome. These are substantial subjects resulting from a recording principle of the MRAM, that is, resulting from a method for reversing magnetization by a current magnetic field generated from each wire.
As one method to overcome these subjects, recording independent of a current magnetic field, that is, a magnetization reversal method, has been investigated. In particular, researches on spin torque magnetization reversal have been actively performed (for example, see Japanese Patent Unexamined Application Publication Nos. 2003-17782 and 2008-227388, U.S. Pat. No. 6,256,223, Phys. Rev. B, 54, 9353 (1996), and J. Magn. Mat., 159, L1 (1996)).
A storage element of the spin torque magnetization reversal is frequently formed to have a magnetic tunnel junction (MTJ) structure as in the case of the MRAM.
In this structure, a phenomenon is used in which when a spin-polarized electron passing through a magnetic layer in which the magnetization is fixed in a certain direction enters another free magnetic layer (the direction of the magnetization is not fixed), a torque (this is referred to as “spin transfer torque”) is imparted to the free magnetic layer, and when a current equivalent to or more than a certain threshold value is supplied, the magnetization of the free magnetic layer is reversed. Rewriting between “0” and “1” is performed by changing the polarity of the current.
An absolute value of the current for this reversal is 1 mA or less for an element having a scale of approximately 0.1 μm. Furthermore, since this current value decreases in proportion to the element volume, scaling can be performed. In addition, since a word line of the MRAM necessary to generate a current magnetic field for recording is not necessary, the cell structure can be advantageously simplified.
Hereinafter, an MRAM using the spin torque magnetization reversal is referred to as a spin torque-magnetic random access memory (ST-MRAM). In addition, the spin torque magnetization reversal may also be called spin injection magnetization reversal in some cases.
As a nonvolatile memory capable of realizing reduction in power consumption and increase in capacity while maintaining advantages of the MRAM, that is, a high operation speed and an approximately infinite number of times of rewriting, great expectations have been placed on the ST-MRAM.
A schematic view of the ST-MRAM is shown in FIGS. 7 and 8. FIG. 7 is a perspective view, and FIG. 8 is a cross-sectional view.
In a portion of a semiconductor base 60, such as a silicon substrate, isolated by an element isolation layer 52, a drain region 58, a source region 57, and a gate electrode 51, which form a selection transistor for selecting each memory cell, are formed. Among those mentioned above, the gate electrode 51 also functions as a word line extending perpendicular to the plane of FIG. 8.
The drain region 58 is formed in common for selection transistors located at right and left sides in FIG. 7, and a wire 59 is connected to this drain region 58.
In addition, a storage element 53 which has a storage layer in which the direction of the magnetization is reversed by the spin torque magnetization reversal is arranged between the source region 57 and a bit line 56 arranged thereabove to extend in a right-to-left direction in FIG. 8.
This storage element 53 is formed, for example, of a magnetic tunnel junction element (MTJ element). The storage element 53 has two magnetic layers 61 and 62. Of the two magnetic layers 61 and 62, one magnetic layer is used as a magnetization fixed layer in which the direction of the magnetization is fixed, and the other magnetic layer is used as a magnetization free layer, that is, a storage layer, in which the direction of the magnetization is changed.
In addition, the storage element 53 is connected to the bit line 56 and the source region 57 with top and bottom contact layers 54, respectively, provided therebetween. Accordingly, when a current is allowed to flow in the storage element 53, the direction of the magnetization of the storage layer can be reversed by spin injection.